Our earlier work showed the hatchetfish giant synapse to be uniquely valuable for study of central synaptic transmission. Both pre-and postsynaptic elements can be independently penetrated by microelectrodes close to the synapse, quantal PSPs can be recorded, and graded release of quanta can be evoked. Some of the synapses can be localized under visual control to within 50 microns. The presynaptic vesicles are depleted by a short period of rapid stimulation and PSPs are greatly reduced in amplitude, yet transmission continues without failures. These data are interpreted as resulting from release of the contents of partially filled vesicles. We plan to analyze the processes of release, fatigue and recovery, both physiologically and morphologically. We will determine the time course of vesicle filling by analysis of evoked quantal sizes during recovery from a depleting tetanus. We will correlate presynaptic stores of vesicles with integrated PSP amplitude during depletion to estimate number of vesicles per quantum. By application of tracer techniques we will further characterize the processes of membrane recycling. Because quanta can be recorded, Ca ions injection presynaptically close to a synapse should give readily detectable transmitter release. Actions and interactions of divalent cations will be studied. Extracellular application or intracellular injection of agents such as hemicholinum and choline should allow analysis of packaging and release of transmitter. Voltage clamping presynaptically will determine the input-output relation of the synapse. Voltage clamping postsynaptically will define the PSP reversal potential and may reveal voltage dependence of synaptic conductance changes. Habituation of the Mauthner cell mediated startle response involves reduced excitability of spinal motoneurons. The mechanisms will be analyzed. We will determine if habituation also occurs at the Mauthner cell itself.